COINCIDENT CONDUCTIVE AND REFLECTIVE MIDDLE AND LOWER CRUST IN SOUTHERN BRITISH-COLUMBIA

Processing and interpretation of magnetotelluric data, recorded as par t of the LITHOPROBE Southern Cordillera transect studies, across the b oundary of the Intermontane and Omineca morphogeological belts reveals : (a) high electrical conductivity in the middle and lower parts of th e crust everywhere, and (b) a depth dependency of geoelectric strike. The data have been modelled using two different inversion algorithms a nd different methods for correcting 'static shifts'. The two different approaches gave similar results: the depth to the top of a conductive layer decreases from 15-17 km in the west across the Intermontane Bel t to 8-10 km across the transition to the Omineca Belt. The top of thi s conductive layer is closely coincident with a layer of increased sei smic reflectivity as shown by reprocessing of collocated LITHOPROBE se ismic-reflection data. The eastward shallowing is associated with an i ncrease in heat flow such that the top of the conductive and reflectiv e zones remains at 400-450-degrees-C. This coincidence suggests that t he increased reflectivity and the high electrical conductivity observe d in the middle crust may have a common cause, and that their presence is limited to where the present temperature exceeds a critical value. One explanation that meets these conditions is that both the conducti vity and reflectivity are produced by a small amount of aqueous fluid porosity. We propose that fluids are trapped in the middle crust by a ductile shear zone, previously interpreted from the seismic sections a s the Okanagan Valley Fault to the west of Okanagan lake. The geoelect rical strike varies from N25-degrees-W for the first 5-10 km of the cr ust, to N20-degrees-E for the middle/lower crust, and to N60-degrees-E for the upper mantle. This variation indicates that the exotic terran e material is concentrated in the uppermost part of the crust and that the remainder of the crust is composed of ancestral North American ro cks.